Macro hex_fmt_impl

macro_rules! hex_fmt_impl {
    ($reverse:expr, $len:expr, $ty:ident) => { ... };
    ($reverse:expr, $len:expr, $ty:ident, $($gen:ident: $gent:ident),*) => { ... };
}

Adds hexadecimal formatting implementation of a trait $imp to a given type $ty.